I just have experienced a very similar problem (the Settings app immediately crashes on opening, de-facto making any kind of access to phone settings impossible). Since this is the first hit for such a question on Google, I'll describe my case and the fix here:
The problem in my case was that the Settings app somehow depends on the Google app and crashes when the Google app is up-to-date (at the time of writing this). It is certainly the case that my phone's Android version is rather ancient (9) and could be due to some oversight on the manufacturer's side, but this nevertheless is a fairly recent regression and Google will hopefully fix it in the future.
As a temporary fix, resetting the Google app to factory via "uninstall" + prohibiting auto-updates for the Google app works.
